One of the goals of plant genetic engineering is to produce plants with agronomically desirable characteristics or traits. Promoters are nucleic acid molecules which comprise the 5′ regulatory elements that play an integral part in the overall expression of genes in living cells. Isolated promoters that function in plants are useful for controlling the expression of operably linked transgenes and thereby modifying plant phenotypes through the methods of genetic engineering.
Many constitutive promoters are available and are useful for providing good overall expression of an operably linked transgene. Examples of such promoters are P-FMV, the promoter from the 35S transcript of the Figwort mosaic virus, (U.S. Pat. No. 6,051,753); P-CaMV 35S, the promoter from the 35S RNA transcript of the Cauliflower mosaic virus, (U.S. Pat. No. 5,530,196); P-Rice Actin 1, the promoter from the actin 1 gene of Oryza sativa, (U.S. Pat. No. 5,641,876); and P-NOS, the promoter from the nopaline synthase gene of Agrobacterium tumefaciens. These promoters provide some level of gene expression in most or all of the tissues of a plant during most or all of the plant's lifespan. Alternately, many promoters are available with more limited expression patterns and are characterized by their tissue specificity, temporal specificity, or developmental specificity. These promoters are useful for the targeted expression of an operably linked transgene in plants. Ultimately, optimal expression of a transgene for producing plants with agronomically desirable characteristics or traits may require such targeted expression. Such targeted expression often requires a promoter having a specific expression pattern which may not be readily available in known promoters.
An example of a desirable targeted expression pattern which is not readily available in known promoters is expression in the maize endosperm during the early stages of kernel development. Endosperm cell division occurs from 5 to 15 days after pollination (DAP). A promoter having an expression pattern active from 5 to 15 DAP would be useful for driving expression of a transgene for a variety of purposes such as preventing kernel abortion in response to stress; promoting cell division to increase endosperm cell number which may increase kernel size and therefore increase grain yield; and altering kernel composition to increase or decrease components such as oil, protein, or starch. The specificity of the promoter for endosperm expression is also important, as ectopic activity in non-targeted tissues has been reported to be associated with the disruption of pattern formation and organogenesis, resulting in deleterious plant growth and development (Meijer M, Murray J A. “Cell cycle controls and the development of plant form.” Current Opinion in Plant Biology 2001 February 4(1):44-49).
Currently characterized cereal endosperm promoters are mainly those of grain storage proteins or those involved in carbohydrate metabolism both of which are turned on at relatively late stages during kernel development (Russell D A, Fromm M E. “Tissue-specific expression in transgenic maize of four endosperm promoters from maize and rice.” Transgenic Research 1997 March 6(2):157-168). The present invention provides a novel promoter capable of driving expression of an operably linked transgene in the endosperm during the early stages of kernel development.